A lithium-ion cell with advantageous features, such as a high energy density and a high operating voltage, has been increasingly used in electronics/communication devices, such as portable phones, digital cameras and personal computers. Further, the lithium-ion cell is expected to be utilized as large-capacity power sources for a satellite, a rocket, an electric vehicle and a nighttime-power storage-based power-load leveling system.
In lithium-ion cells, a carbon-based material capable of absorbing and releasing lithium ions, a lithium-transition metal oxide capable of absorbing and releasing lithium ions, and a nonaqueous solvent such as a carbonate-based solvent with a lithium salt dissolved therein, are used, respectively, in a negative-electrode active material, a positive-electrode active material and an electrolytic solution thereof. Typically, highly crystalline carbon such as graphite, lithium cobaltate (LiCoO2), and a cyclic and/or chain carbonate solvent with lithium fluorophosphate (LiPF6) dissolved therein, are actually used, respectively, in the negative electrode, the positive electrode and the electrolytic solution. The lithium-ion cell is charged in such a manner that lithium is released from the positive electrode into the nonaqueous electrolytic solution, and lithium ions in the nonaqueous electrolytic solution are absorbed into the negative electrode separated from the positive electrode by a micro-porous separator. In a discharge process, the reverse phenomenon is generated to allow electrons to be extracted by an external circuit. Thus, a capacity of the lithium-ion cell is related to a quantity of ions to be absorbed and released between the positive and negative electrodes.
In conjunction with the above charging/discharging reactions, an irreversible decomposition of the nonaqueous electrolytic solution and/or the lithium salt occurs on respective surfaces of the negative and positive electrodes to cause consumption of lithium ions to be absorbed and released. A quantity of lithium ions consumed in the charge/discharge cycles corresponds to an irreversible capacity of the cell. Particularly, during a charge process in the first cycle, a passive film, so-called “solid electrolyte interphase (SEI)”, is formed on a surface of the carbon-based negative electrode, and the resulting irreversible capacity has a great impact on an energy density of the lithium-ion cell. Further, a reductive decomposition of the electrolytic solution occurs on the surface of the negative electrode in proportion to a contact area between the negative electrode and the electrolytic solution. Thus, if the contact area between the negative electrode and the electrolytic solution becomes larger due to expansion of the active material particles of the negative electrode during the charge process, the irreversible capacity will be undesirably increased. When the first-cycle irreversible capacity becomes larger, a ratio of a charge capacity to a discharge capacity (i.e. charge/discharge efficiency) will become lower, and a cell capacity in the second and subsequent cycles will become smaller. Therefore, minimization of the first-cycle irreversible capacity is essential for achieving a high energy density in lithium-ion cells.
Heretofore, in order to reduce the first-cycle irreversible capacity, there have been proposed the following two mainstream measures. One measure is to incorporate an easily-reducible additive into a nonaqueous electrolytic solution. During a charge process, this type of additive can form an unstable passive film on a surface of a negative electrode before a nonaqueous solvent and a lithium salt are reductively decomposed on the negative-electrode surface. This passive film has an electrical insulation performance, and serves as means to block an electrical contact between the negative electrode and the nonaqueous solvent/lithium salt. This makes it possible to suppress the reductive decomposition of the nonaqueous solvent and the lithium salt on the negative-electrode surface so as to reduce a first-cycle irreversible capacity of a cell. For example, Japanese Patent Laid-Open Publication Nos. 2000-348768 and 2000-294282 disclose a technique of incorporating an additive consisting of a nitrate ester, such as isopropyl nitrate, or a sulfite ester, such as alkyl sulfite, into a nonaqueous electrolytic solution, so as to improve a first-cycle charge/discharge efficiency of a lithium-ion cell.
The other measure is to incorporate directly into a carbon-based negative electrode an additive consisting of a different type of carbon-based material, so as to suppress a volume expansion of the carbon-based negative electrode during a charge process to reduce a first-cycle irreversible capacity. For example, Japanese Patent Laid-Open Publication No. 11-73965 proposes a technique of adding fluorocarbon to a carbon-based negative electrode so as to reduce a first-cycle irreversible capacity of a lithium-ion cell.
However, the above two measures are not sufficient to adequately reduce a first-cycle irreversible capacity, and by no means satisfactory from a practical standpoint.
Thus, there is a strong need for providing a nonaqueous electrolytic solution containing an additive capable of suppressing a first-cycle irreversible capacity, based on a new concept from a standpoint of improving energy density and charge/discharge cycle characteristics of a lithium-ion cell.